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sungrazing comet
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Sun's surface. Although small sungrazers can completely evaporate during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong evaporation and tidal forces they experience often lead to their fragmentation.
Up until the 1880s it was thought that all bright comets near the sun were the repeated return of a single sungrazing comet. Then German astronomer Heinrich Kreutz and American astronomer Daniel Kirkwood determined that instead of the return of the same comet, each appearance was a different comet but each were related to a group of comets that had separated from each other at an earlier passage near the sun (at perihelion). Very little was known about the population of sungrazing comets until 1979 when coronagraphic observations allowed the detection of sungrazers. As of December 12, 2013 there are 1488 known comets that come within ~12 solar radii (~0.055 AU).〔(JPL Small-Body Database Search Engine )〕 This accounts for nearly one third of all comets. Most of these objects vaporize during its close approach but a comet with a nucleus radius larger than 2–3 km is likely to survive the perihelion passage with a final radius of ~1 km.
Sungrazer comets were some of the earliest observed comets because they can appear very bright. Some are even considered Great Comets. The close passage of a comet to the sun will brighten the comet not only because the reflection off of the comet nucleus brightens when it is closer to the sun, but the sun also vaporizes a large amount of gas from the comet and the gas reflects more light. This extreme brightening will allow for possible naked eye observations from Earth depending on how volatile the gases are and if the comet is large enough to survive perihelion. These comets provide a useful tool for understanding the composition of comets as we observe the outgassing activity and they also offer a way to probe the effects solar radiation has on other solar system bodies.
== History of Sungrazers ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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